Modules for converting solar energy into useful forms of energy such as heat and electricity have been in existence for many years. Because of the sun's low energy intensity and the moderate conversion efficiency of solar modules, a large array of solar modules is often required to service the end-use of the energy. Array areas from several dozen square feet to several thousand square feet are common. Moreover, the variety of surfaces on which the modules may be mounted requires a wide range of flexibility and adaptability in the mounting hardware that will be used to structurally anchor the modules to the mounting surface.
An example of a solar module is a solar photovoltaic (PV) module, which consists of a series of PV cells connected in a series and parallel combination to yield a specific current and voltage output. Due to the fragility of the cells and the harsh environmental conditions they are often exposed to, the assembly of cells is often encapsulated into a rigid laminate. Most PV laminates are fabricated from a glass cover, an active layer containing the PV cells, and a back cover. While PV laminates can be directly attached to a mounting structure, it is more common for them to be framed before mounting. PV laminate frames typically consist of aluminum extrusions with an upper cavity that receives the laminate when assembled. The frame serves the purpose of increasing the rigidity of the laminate and to protect the fragile glass edge of the laminate from cracking. Frames for PV modules often include a lower flange with pre-drilled holes for affixing them to mounting structures.
Because PV modules must be electrically interconnected, they are often mounted in strings where the modules are assembled end to end to form a row of modules. Due to the fact that most mounting surfaces such as roofs are square or rectangular in nature, most PV module installations consist of multiple rows assembled in close proximity to match the general footprint of the surface on which they are mounted. Such arrangements of multiple rows of modules are generally referred to as an array.
Dual Strut Runner (DSR) Technique
A further example of a method for attaching rows of framed PV modules to a surface is through the use of Dual Strut Runners (DSR). In a DSR system, the PV modules are affixed to a pair of strut style rails that run the length of the assembled row. These rails are in turn affixed to the mounting surface through individual footings. The strut style rails used in this type of mounting may be either common channel style strut such as UNISTRUT™ of Wayne, Mich. or B-Line™ of Highland, Ill. or proprietary strut style rail made explicitly for mounting solar modules. Two examples of strut style rails made explicitly for mounting framed PV modules are described in US Published Patent Application Nos. 2003/0015636 A1 and 2002/0046506 A1 and are herein incorporated by reference in their entirety. The DSR system may also be employed with unframed PV laminates as disclosed in US Published Patent Application No. 2003/0070368 A1 and herein incorporated by reference in its entirety.
Two mounting methods are possible with most DSR systems. The first method is to attach the rails to the mounting surface with footings first and to then attach the PV modules to the rails with a front-side mounting technique. The second method is to first mount a row of modules onto the rails from the back-side to form a rigid sub-assembly consisting of multiple PV modules and then lift the entire sub-assembly onto the mounting surface to be attached. Both of these techniques have their benefits and drawbacks as outlined below.
Using the front-side mounting technique with DSR systems requires access to the rails such that mounting clips may be secured. If the PV modules were to directly abut one another then the rails would be obscured underneath them with no possible attachment points. To remedy this issue, a small gap on the order of 1″ is typically left in between abutting modules to allow a fastening clip to be installed. The purpose of the fastening clip is to compress the PV module frames against the underlying rails. Although it is easy to attach the modules in this manner, a disadvantage of the front-side mounting technique is that it results in high pressures for a given clip compression. These high pressures can compromise the integrity of the module frame or the glass laminate and place an upper limit on practical clamping forces. Another disadvantage of this method is that visible gaps are left between the modules as well as the discrete clips, both of which ruin the visual aesthetics of the installation.
Many DSR systems may also be mounted using a back-side technique. With this technique, the rails are first attached to the back-side of the frames on the PV modules, often using holes supplied by the manufacturer in the PV module frame. Because access to the back-side of the modules is possible, there is no need for access from the front-side, and adjacent modules may therefore directly abut one another without a resultant gap. After a series of modules are attached to the strut runners to create a row, the entire row of modules is then lifted up onto the mounting surface to be attached to the footings. The primary advantage of the back-side technique is that the modules may be mounted in a seamless manner without the gap that results between modules with the front-side method. A primary disadvantage of the back-side method is that large assemblies of modules must be lifted onto the mounting surface. Whereas an individual may be able to lift and arrange single modules with the front-side method, several individuals and mechanical equipment is often required to lift and arrange entire rows as is necessary with the back-side method. A second disadvantage of the back-side method is that the strut runner positions are set when the subassembly is created and those positions must exactly match the footings already installed on the mounting surface. Two separate sub-assemblies are created that must have an exact match fit when fully assembled.
Common Compressed Rail Technique
There exists a need for a mounting system, method and apparatus for solar modules that would allow for the aesthetically preferred seamless mounting possible with the DSR back-side technique, but with the ease of installation that comes with the DSR front-side technique. The most practical method of attaining this ideal mounting technique is to employ a Common Compressed Rail (CCR) as the mounting structure that attaches the modules to the footings on the mounting surface.
The CCR method is quite common in the architectural glass industry for assembling monolithic curtain walls in high-rise buildings. The CCR method has been disclosed in U.S. Pat. Nos. 4,223,667 and 6,105,317, both of which are herein incorporated by reference in their entirety. The principal difference between the DSR and CCR methods is that the DSR method consists of a pair of rails placed underneath a row of modules while the CCR method consists of a common rail that bridges adjacent rows of modules.
By placing the mounting rail between rows of modules instead of underneath them, the CCR method allows the modules in a single row to seamlessly abut one another while still allowing access to the rail such that clamping of the modules may be attained.
In its most prevalent form, the CCR system consists of a lower rail that is affixed to the mounting surface. The lower rail contains a pair of extended shelves or surfaces to accept the solar modules that will rest on both the left and right sides of the rail. Once the solar modules have been assembled in the lower rails, an upper rail or cap that is designed to mate with the lower rail will be put in place and tightened against the lower rail. With the edges of the modules held between the upper and lower rails, the tightening of the upper rail compresses the modules within the assembly and keeps them captive. While the CCR technique overcomes some of the basic issues inherent with the DSR technique, the CCR systems as currently practiced suffer from several disadvantages.
There is currently no set standard for solar module frame construction resulting in a multitude of currently available frame heights with the vast majority falling in the range of 1 inch to 2 inches in height. The issue in racking different frame heights with a CCR system arises at the free edge of the module array. The free edge of the array occurs on the last row of the array where modules will be placed on only one side of the common rail and the other side is open or empty. If the rail set on the free edge were to be simply compressed, the upper and lower rails would begin to fold over on the free and unsupported side without attaining proper orientation and compression on the side containing the actual module frame. To properly compress the module frame, it becomes necessary to ‘fix’ the free edge at approximately the same height as that of the module frame such that further compression of the upper and lower rails results in compression and capture of the module frame.
Current implementations of the CCR technique rely on either a custom interference fit of the upper and lower rails to fix the free edge, or they implement a dummy module frame of the same height as the actual module frames to be racked. Both methods of fixing the free edge of the array are disclosed in U.S. Pat. No. 6,105,317 which is herein incorporated by reference in its entirety.
Using the custom interference fit method, a contact region in designed into the assembled fit of the upper and lower rails such that they contact at the approximate height of the module frames. Once the free edge of the rails has contacted in this manner, further compression results in clamping of the module frames with the contacted edge acting as a pivot. The disadvantage of this method is that a single rail combination can only accommodate a fraction of an inch in module frame height variation before the rails begin to significantly fold over on the free edge during compression. It therefore becomes necessary to create multiple rail sets to accommodate any significant variation in module frame heights.
The dummy frame method requires an empty frame with approximately the same height as the module frames. The dummy frame is installed on the free edge opposite the actual PV module frame. Because of the symmetry in frame heights, the rails will compress uniformly on both frames. This method incurs the same disadvantage as the custom interference fit in that each dummy frame can only accommodate a fraction of an inch in module frame height variation before another dummy frame is required. It is therefore necessary to create multiple dummy frames to accommodate any significant variation in module frame heights.
A further disadvantage common to the above described CCR system is the design of the upper rail section. Presently available CCR systems place the majority of their inertial mass and therefore rigidity in the lower rail section. The upper rail is often designed merely as a cosmetic capstrip with low inertial mass just sufficient to compress the module frame against the lower rail. The low inertial mass of the upper rail is insufficient to spread the force of the member that compresses the rails and therefore results in significant point loads on the PV module frames. The low inertial mass of the upper rail also compromises the rigidity of the compressed assembly and its ability to resist loading by wind and snow.